In general, cemented carbide (a WC—Co alloy or an alloy prepared by adding a carbonitride of Ti (titanium), Ta (tantalum) or Nb (niobium) thereto) has been employed for a tool for cutting. Following the recent speed increase in cutting, however, the usage of a hard alloy tool comprising a substrate of cemented carbide, cermet, a cubic boron nitride sintered body or alumina- or silicon nitride-based ceramics covered with at least one coating layer of a compound composed of at least one first element selected from group IVa elements, group Va elements and group VIa elements of the periodic table of elements, Al (aluminum), Si and B and at least one second element selected from B, C, N and O (if the first element is only B, the second element is not B) on the surface thereof by CVD (Chemical Vapor Deposition) or PVD (Physical Vapor Deposition) with the coating layer having a thickness of 3 to 20 μm increases.
Such a cutting tool has a rake face coming into contact with chips of a workpiece and a flank face coming into contact with the workpiece itself, and a portion corresponding to an edge where the rake face and the flank face intersect with each other (and a portion around the same) is referred to as an insert edge.
In recent years, the cutting speed has been further increased in order to further improve cutting machinability, to require higher wear resistance to such a cutting tool. However, the requirement for high wear resistance leads to reduction of toughness, and hence compatibility between high wear resistance and high toughness is required.
As an attempt to satisfy this requirement, Japanese Patent Laying-Open No. 05-177411 (Patent Document 1), for example, pays attention to residual tensile stress of a coating layer caused when the coating layer is formed on a substrate at a high temperature by chemical vapor deposition (CVD) and thereafter cooled to the room temperature, assumes that this tensile stress reduces toughness of a tool and proposes a measure for solving this problem. While this tensile stress results from the difference between the thermal expansion coefficients of the substrate and the coating layer, a technique of first forming a first coating layer having such tensile stress on the substrate, forming prescribed cracking on this first coating layer and thereafter forming a second coating layer having compressive stress on the first coating layer thereby improving toughness (fracture resistance) while maintaining high wear resistance is employed.
Japanese Patent Laying-Open No. 05-177412 (Patent Document 2), paying attention to tensile stress of a coating layer similarly to the above, employs an approach different from the above and proposes a structure obtained by forming an inner coating layer having tensile stress on a hard ceramics substrate and forming an outer coating layer having compressive stress thereon. Further, Japanese Patent Laying-Open No. 05-177413 (Patent Document 3) proposes a cutting tool, comprising a substrate of cermet, having a structure similar to that of Patent Document 2.
On the other hand, Japanese Patent Laying-Open No. 06-055311 (Patent Document 4) proposes a cutting tool prepared by forming a hard coating layer on a substrate of cemented carbide by chemical vapor deposition and substantially removing tensile stress from a rake face portion of the hard coating layer while holding tensile stress of a flank face portion of the hard coating layer.
Japanese Patent No. 3087465 (Japanese Patent Laying-Open No. 06-079502, Patent Document 5) proposes a cutting tool prepared by forming a hard coating layer having a substantially identical compressive stress distribution along the overall insert edge on the surface of a titanium carbonitride-based cermet substrate and performing shotblasting on this hard coating layer thereby rendering compressive stress of a rake face portion greater than compressive stress of a flank face portion by at least 49 MPa.
While toughness and wear resistance can be rendered compatible to some extent in each of the aforementioned proposals, however, higher performance is required to a cutting tool under the present circumstances around the cutting tool, and development of a cutting tool sufficiently satisfying such performance is demanded.    Patent Document 1: Japanese Patent Laying-Open No. 05-177411    Patent Document 2: Japanese Patent Laying-Open No. 05-177412    Patent Document 3: Japanese Patent Laying-Open No. 05-177413    Patent Document 4: Japanese Patent Laying-Open No. 06-055311    Patent Document 5: Japanese Patent No. 3087465 (Japanese Patent Laying-Open No. 06-079502)